If you were previously successfully communicating with your MegaSquirt ECU, the firmware is likely corrupted. While extremely poor wiring and severe electrical noise could corrupt the firmware, the more likely culprit is an improperly configured serial connection, or tuning software. Besides ensuring your serial connection is sound, you will have to flash your ECU with the latest available firmware to restore functionality. Be sure the firmware in your ECU matches the .INI file your tuning software is using, otherwise the problem will recur.
Please review these articles on properly configuring your serial connection and flashing your ECU:
Please feel free to contact SymTech Labs' support team if you cannot restore communications after taking these steps.